This application is based on Japanese Patent Application No. 10-310146 filed Oct. 30, 1998, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a charging roll for use in an image forming apparatus such as an electrophotographic copying machine or printer.
2. Discussion of the Related Art
There is known a charging roll which is installed in an image forming apparatus such as an electrophotographic copying machine or printer, such that the charging roll is held in rolling contact with a photosensitive drum for charging the circumferential surface of the photosensitive drum. More specifically described, such a charging roll is used in a so-called xe2x80x9croll chargingxe2x80x9d method which is one of the known methods for charging a photosensitive drum on which an electrostatic latent image is formed. In the roll charging method, the charging roll to which a charging voltage is applied is held in pressing contact with the outer circumferential surface of the photosensitive drum. The charging roll and the photosensitive drum are rotated together so that the outer circumferential surface of the photosensitive drum is evenly charged by the charging roll.
Since the charging roll is held in rolling contact with the outer circumferential surface of the photosensitive drum, the charging roll is required to have a low hardness (high softness) and high flexibility. Further, the charging roll is required to have a suitable degree of electric conductivity, so as to effectively charge the photosensitive drum.
In recent years, there is an increasing demand for higher image forming capability and excellent energy-saving characteristics of the image forming apparatus, as well as higher process speed and excellent durability of the image forming apparatus. In an attempt to improve the energy-saving characteristics for reducing the electric power to be consumed by the image forming apparatus, the melting point of the toner is lowered for the purpose of fixing the toner on a recording medium at a lower temperature. Further, for improving the performance of the image forming apparatus to provide sufficiently high image quality, the size of the toner particles is made smaller.
However, when the melting point of the toner is lowered or the size of the toner particles is made smaller, the toner undesirably tends to adhere to the charging roll which is inevitably heated in the image forming apparatus during its operation. The toner which adheres or clings to the charging roll causes deterioration of the image forming capability of the image forming apparatus. Described more specifically, with an increase in the number of copying or printing operations, in other words, with an increase in the number of operations to develop the electrostatic latent images into visible toner images on the outer circumferential surface of the photosensitive drum, the electric resistance of the charging roll tends to be raised as a whole due to the adhesion of the toner to the charging roll. Further, uneven adhesion of the toner to local portions of the roll causes a variation in the electric resistance of the roll at the local portions. As a result, the image to be formed or reproduced by the image forming apparatus is undesirably deteriorated.
Recently, there is an increasing demand for an image forming apparatus capable of reproducing a color image. In the reproduction of the color image, a non-magnetic one-component toner is generally used to establish a desired hue or color. The non-magnetic one-component toner tends to be easily charged in the absence of a magnetic component such as magnetite or ferrite therein, which serves as a black-color pigment. Thus, for controlling the amount of charging of the toner, a large amount of finely particulate additives such as SiO2, TiO2 or Al2O3 are added to the toner.
Conventionally, a suitable amount of carbon black or other electrically conductive agent is added to a material which constitutes a surface layer (outermost layer) of the charging roll, so that the surface layer has a desired value of electric resistance. In general, the surface layer of the charging roll is required to have a volume resistivity within a range of about 107-1015 xcexa9xc2x7cm. For controlling the volume resistivity within such a limited range, there is proposed the use of an electrically conductive agent which does not excessively influence or reduce the electric resistance, so that the electric resistance does not greatly change with a change of the amount of the conductive agent to be added to the material of the surface layer. Thus, the use of such an electrically conductive agent permits the volume resistivity to be finely controlled by adding a large amount of the electrically conductive agent to the material of the surface layer, without necessity of a delicate adjustment of the amount of the conductive agent to be added. However, the carbon black or the large amount of the conductive agent added to the material of the surface layer leads to an increased size of the secondary particles of the carbon black and an increased amount of the conductive agent exposed on the surface of the charging roll. Thus, the surface of the charging roll is activated and considerably roughened with a multiplicity of minute recesses and projections, due to the increased size of the secondary particles and the increased amount of the exposed conductive agent, whereby the toner and additives included in the toner are more likely to stick or adhere to the activated surface of the exposed conductive agent, or to accumulate in the recesses of the rough surface of the charging roll, causing the above-described variation in the electric resistance at the local portions of the surface of the roll, and accordingly making it difficult to establish an even distribution of the electric resistance over the entire surface area of the charging roll. As a result, the image formed or reproduced by the image forming apparatus is likely to be problematically deteriorated.
It is therefore an object of the present invention to minimize a local variation of the electric resistance value of the surface layer of a charging roll, by preventing adhesion or accumulation of toner and additives to or on the outer surface of the charging roll, for preventing deterioration of an image formed by an image forming apparatus on which the charging roll is installed.
It is another object of this invention to provide a charging roll which has an improved ease of control of the electric resistance value, a sufficiently even distribution of the electric resistance value of the surface layer of the charging roll, and improved smoothness of the outer surface of the charging roll.
The above objects may be achieved according to the principle of the present invention which provides a charging roll that is held in rolling contact with a photosensitive drum for charging the photosensitive drum. The charging roll includes an outermost layer formed of a resin composition containing a grafted carbon as an electrically conductive agent. The grafted carbon includes a carbon black and a polymer which is grafted on the carbon black so as to cover a surface of the carbon black.
In a conventional charging roll, a carbon black is used as an electrically conductive agent which is a constituent of the resin composition forming the outermost layer. In the charging roll constructed according to the present invention, on the other hand, the electrically conductive agent is constituted by the grafted carbon which is obtained by grafting the suitable polymer on the carbon black. The carbon black is coated with the grafted polymer, and active points or spots on the surface of the carbon black are accordingly covered by the grafted polymer. This covering of the active spots by the grafted polymer advantageously provides improved dispersibility of particles of the carbon black and also a minimized agglomeration or cohesion of the carbon black particles, leading to a sufficiently even distribution of the electric resistance value and improved smoothness of the outer surface of the charging roll. Further, the covering of the surface of the carbon black by the grafted polymer provides other advantages that the ease of control of the electric resistance is improved, and that there is no risk of chemical bonding between the carbon black and the toner even where the grafted carbon as the electrically conductive agent is exposed on the outer surface of the roll. Therefore, the arrangement according to the principle of the invention is effective to facilitate removal of the toner and additives from the outer surface of the roll, and accordingly prevent or minimize adhesion or accumulation of the toner and additives to or on the surface of the roll.
According to a first preferred form of the present invention, the weight ratio of the polymer which is grafted onto the carbon black, to the carbon black in the grafted carbon is 0.2-1.0, thereby making it possible to maximize the advantages provided by the use of the grafted carbon as the electrically conductive agent.
According to a second preferred form of the invention, wherein a content of the grafted carbon in the resin composition is 5-50 wt. %, thereby increasing the electric conductivity of the outermost layer and advantageously establishing a desired value of the electric resistance.
According to a third preferred form of the invention, the charging roll has a surface which is constituted by an outer surface of the outermost layer and which has a surface roughness: Rz (ten point height of irregularities) of not larger than 3 xcexcm, whereby the adhesion or accumulation of the toner and additives to or on the surface of the roll is more effectively prevented or minimized.
According to a fourth preferred form of the invention, the outermost layer has a volume resistivity within a range of 107-1015 xcexa9xc2x7cm, so that the charging roll effectively functions as a charging roll.
According to a fifth preferred form of the invention, the resin composition forming the outermost layer includes at least a fluorine-modified acrylate resin as a resin component of the resin composition.
According to one advantageous arrangement of the fifth preferred form, the resin composition further includes at least one of a fluorinated olefin resin and a fluorine-unmodified acrylate resin in addition to the fluorine-modified acrylate resin.
In the charging roll according to this fifth preferred form, it is possible to assure easier removal of the toner and additives from the surface of the roll, and further effectively minimize or prevent the adhesion or accumulation of the toner or additives to or on the surface of the roll.
According to a sixth preferred form of the invention, the charging roll further includes a center shaft, a soft base layer formed on an outer circumferential surface of the center shaft, a resistance adjusting layer formed radially outwardly of the soft base layer, and a protective layer formed outwardly of the resistance adjusting layer and serving as the outermost layer.
According to one advantageous arrangement of the sixth preferred form, the soft base layer consists of a solid elastic body which is electrically conductive.
According to another advantageous arrangement of the sixth preferred form, the soft base layer consists of a foam body which is electrically conductive.
According to a seventh preferred form of the invention, the charging roll further includes a softener-blocking layer interposed between the soft base layer and the resistance adjusting layer, whereby bleeding of a softener such as an oil from the soft base layer is minimized or prevented by the softener-blocking layer.